Broad band bidirectional antenna



July 21, 1953 A. ALFORD BROAD BANDBIDIRECTIONAL ANTENNA Filed March*l,1945 INVENTOR ANDREW ALFORD & AAA

ATTORNEY Patented July 21, 1953 BROAD BAND BIDIRECTIONAL ANTENNA AndrewAlford, Cambridge, Mass, assignorto, the

United States of America asrepresented by the Secretary of WarApplication March 1, 1946, SerialNo.'651,-3l6

2 Claims. (01. 250-4351) This invention relates generally to electricalapparatus and more particularly to antennas.

The conventional dipole radiator, which isone of the most simple formsof antennas, has a bidirectional free space pattern generally resemblingthe figure eight. The front and rear lobes are equal in shape andresponse, and complete nulls exist at both sides.

In certain applications it is frequently desirable to use an antennawhose pattern is generally unidirectional but whose side nulls do notdrop to a value of zero. That is, it may be required that the principallobes be forward and to the rear but with some response from the sides.

It is well known that an antenna array can be designed which will haveany pattern desired, within reason. However such, arrays usually becomecomplex in proportion to the requirements of the antenna pattern andsuch an array is not acceptable for use in many applications, such as inaircraft for example.

It is an object of the present invention to provide a dipole antennacapable of some response from the sides.

It is another object to provide a generally bidirectional antennacapable of operation over a wide range of frequencies.

It is also an object to provide a generally bidirectional, broad band.antenna particularly adapted for use on aircraft.

An antenna which will accomplish the above objects, consists of a bent,sleeved, plane p'olarized, dipole with an impedance balancing devicegenerally known as a"balun. A conventional dipole radiator is bent sothat the longi-- tudinal axes of the radiating elements no longer arecommon but intersect to form a V. An inner portion of the dipole isenclosed in a sleeve and a balun device is connected between the twodipole elements and a coaxial feed line. The balun is so called becauseit provides a means for connecting a system which is balanced withrespect to ground to a system which is unbalanced with respect toground.

Other objects, features and advantages of this invention will suggestthem-selves to those skilled in the art and will become apparent fromthe following description of the invention taken in connection with theaccompanying drawing which is an isometric view of an antenna embodyingthe principles of this invention.

Referring now to the single figure of the drawing, two conductorelements I and. II with their respective sleeves, I2 and I3 are bent toform a V.. Theincluded angle 0. of .the .V, as shown inthed-rawing, isabout degrees. Perpendicular to the plane formed by the radiatingelements, a balunI 4 is connected'to conductor elements I0 and I I .andsleeves I2 and I3.

.-Balun I4, consists of an outer sheath I5, impedance balancingconductor I6, a coaxial transmission line I! and an end plate I8.Conductor I6 is contained in sheath l5. One end of conductor I6 isconnected to end plate I8 andits other end is connected to an end ofconductor II]. A portion of coaxial line I1 is also contained in sheathI5 and the outer conductor of line I! is shorted to end plate l8 Whereit enters sheath I5. The outer conductor of line I! terminates incontact with an end of conductor" element II. Near the termination ofthe outer conductor of line Ii, the inner conductor of line I1 leavesthe confinement of its outer conductor and terminates in contact withthe juncture of conductor I6 and conductor ID. A portion of coaxial lineI! 1 I5 for connection purextends outside of sheath poses.

.All previously mentioned components of the antenna assembly aremetallic conductors! Some nonconductive form of supporting material isrequired to maintain the inner and outer conductors of line I! in theirusual concentric orientation, however, such a support being conventionalis not of importance in the disclosure of this invention and has beenomitted in the drawing for clarity.

The antenna above described is a'balanced system in that it consists oftwo symmetrical conductors I0 and II, which are energized by equal andopposite voltages and conductors I0 and I'I'are at an impedance balancewith respect to ground. Conductor I0 is connected to the inner conductorof line H and conductor I I is connected to the outer conductor of lineH, resulting in equal and opposite voltages being applied to the tworadiators. Sheath I5 is at ground potential and each of conductors I0and I I is connected to end plate I8 of sheath I5 by a quarterprove theinput impedance of the antenna and each has a length of approximatelyone-eighth of a wavelength. Conductors i and H each have a length ofapproximately one-fourth wavelength. Each of conductors l0 and H extendsbeyond their respective sleeves by an amount approximately equal toone-eighth of a wavelength.

Since this antenna will operate satisfactorily over a wide range offrequencies, all the aforementioned lengths relative to wavelengths,correspond to a wavelength whose frequency is approximately the midpointof the operating frequency band.

An angle of 100 degrees between the two branches of the dipole has beenused to increase the amount of side response. However, the antenna stillmaintains a generally bidirectional response pattern with the two majorlobes forward and to the rear. 7

An antenna constructed in accordance with the teachings of thisinvention will operate over a two to one band of frequencies with astanding wave ratio of about two to one or better.

While there has been here described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention.

What is claimed is:

1. A broad band plane polarized generally bidirectional bentsleeveddipole antenna for producing two main lobes of radiation in oppositedirections with some radiation to the sides including a first radiatorhaving a cylindrical shape with a length substantially equal to onefourth of a wavelength corresponding to the midpoint of the operatingfrequency band, a second radiator physically similar to said firstradiator oriented with respect to said first radiator so that thelongitudinal axes of said first and second radiators intersect to form aV, the

included angle of said V being substantially 100 degrees, a firsttubular sleeve having a length substantially equal to one eighth of awavelength corresponding to .the midpoint of the operating frequencyband enclosing a portion of and insulated from said first radiator; asecond tubular sleeve physically similar to said first sleeve enclosinga portion of and insulated from said second radiator, an impedancebalancing system oriented perpendicular to a plane formed bysaidradiators comprising a tubular metallic sheath having an end plate atone end with its open end electrically connected to said sleeves, animpedance balancing conductor enclosed by said sheath electricallyconnected to an end of said first radiator with its other endelectrically connected to said end plate, said impedance balancingconductor having a length substantially equal to one quarter of awavelength corresponding to the midpoint of the operating frequencyband, a coaxial transmission line inserted in said sheath through a holein said end plate with its outer conductor shorted to said end plate andterminated in contact with an end of said second radiator and with theinner conductor of said coaxial transmission line terminated in contactwith the juncture of said impedance balancing conductor and said firstradiator;

2. A broad band plane polarized generally unidirectional bent sleeveddipole antenna for posite directions with some radiation tothe sides Iincluding a first radiator having a cylindrical shape with a lengthsubstantially equal to one quarter of a wavelength corresponding to themidpoint of the operating frequency band, a second radiator physicallysimilar to said first radiator and oriented-with respect to saidfirstradiator so that the longitudinal axes of said first and secondradiators intersect to form a V, the included angle of said V beingsubstantially degrees, a first tubular sleeve enclosing a portion of andinsulated from said first radiator and having a length substantiallyequal to one eighth of "a wavelength corresponding to themidpoint of theoperating frequency band; a second tubular sleeve physically similar tosaid first tubular sleeve enclosing a portion of and insulated from saidsecond radiator, electrical meansconnecting said sleeves to a point ofreference-potential, and a coaxial line balanced to unbalanced systemhaving a length substantially equal to one fourth of a wavelengthcorresponding to the midpoint of the operating frequency band and beingconnected to said radiators whereby'the current distribution on saidradiators is modified so that said antenna has "a lowinput impedance andcan be fed by an unbalanced transmission 'line.

References cited in the file of this patent UNITED STATES PATENTS2,530,048 'Dri'scoll' Nov; 14, 1950 ANDREW LF

